The present invention relates to a transmission system for use in communicating between a base station and a mobile station.
A variety of mobile communication systems, such as mobile telephone or portable telephone for effecting communication between a base station and a mobile station have heretofore been put into practice. The mobile communication system is fundamentally the same communication system as for effecting communication between fixed stations.
In a mobile communication terminal such as mobile telephone or portable telephone, a received signal tends to be distorted due to the influence of so-called multipath fading. Specifically, when the multipath fading occurs, a propagation delay between paths increases to cause an intersymbol interference. As a consequence, preceding and succeeding codes overlap each other so that a transmission characteristic is degraded.
In order to satisfactorily receive a reception signal even when the transmission characteristic is degraded, a sync (synchronizing) detecting circuit formed of an adaptive equalizer or PLL (phase-locked loop) circuit has to be applied to the receiver. Therefore, the receiver becomes complex in arrangement and becomes expensive from a money standpoint.
In order to solve the aforesaid problem, the same assignee of the present application has previously proposed a communication system where information is transmitted on the basis of a phase difference between the carriers according to a so-called multiple carrier system for simultaneously transmitting a plurality of carriers (see Japanese patent application No. 6-216311). This previously-proposed communication system will be described later on in the specification.
The above-mentioned multiple carrier communication system encounters the following problems when a reception signal is received:
1) A timing at which a received wave is demodulated is difficult to be accurately synchronized with a modulation symbol duration of a transmitted signal so that an offset occurs in a demodulation timing. If the offset occurs in the demodulation timing, then an error dependent on a carrier frequency occurs in demodulated phase information. As a consequence, an error rate in judgement increases. Further, if synchronizing data is added to the transmission signal in order to synchronize the demodulation timing with the modulation symbol duration of the transmission signal, there is then the disadvantage that an amount of information to be transmitted is reduced, resulting in a transmission efficiency being lowered; and
2) A frequency offset of a received wave is difficult to be eliminated. If this frequency offset exists, then an error of a constant value occurs in demodulated phase difference information with the result that an error rate in judgement increases.